The present invention relates to the field of medicine, and in particular to the determination of urea levels in blood and serum. More specifically, the invention concerns a novel type of indicator for the simple and rapid determination of urea concentrations.
For determining the concentration of urea in blood and serum samples, use has been made of the enzymatic reaction with urease, the products of which are carbon dioxide and ammonia. In some prior art embodiments, determination of the change in pH is effected in the urease reaction zone of the indicator strip using pH indicator dyes (DE-AS No. 1,498,601 and DE-AS No. 1,598,756). In other systems, measurement takes place in a separate indicator zone of the liberation of ammonia gas) DE-AS No. 1,245,619 and DE-AS No. 2,249,647). Calibration of the indicator compounds may be effected through the use of organic acids. From this data, the urea concentration of the sample may be determined according to the parameters of the urease reaction.
In order to improve the stability of the reaction and of the dyes, known additives have been employed in some prior art embodiments. For example, an alkali buffering system with the addition of organic acid and stabilizing agents is known to be an effective means of increasing the accuracy of determinations.
Increasingly, tests of urea levels are being performed on whole blood or serum in ambulances and at sickbeds. For a quantitative or semiquantitative measurement, an accurate pipetting of between 0.01 and 0.1 ml or sample is required. The indicator strips must be kept in a refrigerator if they are to be used over a long period of time, as they are sensitive to basic contaminants in the air. The indicator zone containing dyes is calibrated using printed or stencilled gauges, but the meniscus effect and the indefiniteness of the measurement line hampers accurate determinations by an inexperienced person.
Added to these drawbacks is the fact that the preparation of indicator strips with numerous reagents is complicated and requires high standards of quality control to insure reproducibility. As the urease activity is markedly reduced by the presence of traces of certain known metals, the purity of the compounds used must also be maintained to a great degree.